Infrared spectroscopy became popular as an analytical tool for the synthetic rubber program of World War II. At the time, synthetic rubber was synthesized by polymerizing butadiene, so a reliable way of assessing the concentration and purity of butadiene was necessary for consistent yields. IR spectroscopy fit the bill perfectly.
Infrared spectroscopy probes the molecular vibrations of molecules. Light of different energies (or frequency, represented by wavenumbers in the spectrum above) is directed through a sample. When a particular energy (or frequency) of light matches a vibrational frequency of the molecule, the molecule absorbs the light and vibrates. A detector monitors the light intensity at each frequency over the entire spectrum. When the sample absorbs light, less light passes to the detector, resulting in a lower percent transmission, or a peak in the infrared spectrum. Peaks in an infrared spectrum are upside-down compared to other forms of spectroscopy to convey that the peak is a decreased intensity, or absorbance of light.
The region of an infrared spectrum below approximately 1600 wavenumbers is known as the fingerprint region. This region of the spectrum is a combination of molecular vibration and possibly rotation that is unique to each molecule but difficult to interpret for structural information. The fingerprint region is useful for identifying specific molecules.
The remaining area, usually from approximately 1600 to 3500 wavenumbers, is used to measure characteristic vibrational absorbances of functional groups, or structural characteristics of a molecule. In the spectrum of butadiene above, the peak at approximately 1660 wavenumbers is characteristic of the C=C (alkene) stretching frequency and the peak at approximately 1800 wavenumbers is characteristic of the C=CH2 stretch found in butadiene.
The large peak centered around 3000 wavenumbers is familiar to anyone who has used infrared spectroscopy. In butadiene, this peak is really two overlapping peaks. Below 3000 wavenumbers, the peak results from the -C-H stretching frequency. Above 3000 wavenumbers, the peak results from -C=CH carbon-hydrogen bond stretching.
Sources
NIST Chemistry Web Book. http://webbook.nist.gov/chemistry/ First accessed 11/00.